To identify and/or monitor a state or event, such as a physiologic state or event, sensed signals or waveforms representative thereof are typically used. Once the relationship between the sensed signals or waveforms and the corresponding physiologic state or event is established, the signals can also be used to diagnose a condition and/or configure therapy delivery systems or methods.
For example, in the area of cardiac monitoring and/or therapy, implantable devices are employed for sensing cardiac signals, diagnosing conditions, and providing therapy. Such devices include electrodes for sensing and sense amplifiers for recording and/or deriving sensed event signals from electrical currents associated with cardiac activity, i.e., intracardiac or remote electrograms (EGMs). Implantable devices which also provide therapy include pacemakers, cardioverter/defibrillators, cardiomyostimulators, neurostimulators, and drug delivery devices. Such implantable devices are programmable and/or can be interrogated by an external programmer through the use of bi-directional radio frequency (RF) telemetry to exchange data and commands via uplink and downlink RF telemetry transmissions through the patient's skin.
Sensed signals detected by the implantable device and transmitted to and received by the external programmer comprise a continuous signal stream characterized by periodic repetition and impulse-like spikes in its waveform shape which correspond to electrical activations of the heart. Since these sensed signals represent a cardiac event or state, the spatial resolution or waveform morphology of a signal and its change over time are extremely valuable because conditions of the heart may be diagnosed and treatments configured based on such waveform morphology. Unfortunately, because the electrodes of implantable devices cannot be completely isolated from electrical currents associated with non-cardiac tissue, e.g., other muscles, and/or due to the sensed signals having to travel through tissue, such as the patient's skin, to be transmitted to the external programmer, it is not uncommon for sensed signals to be susceptible to noise and/or loss of spatial resolution.
With conventional signal processing, input signals are similarly filtered regardless of the actual morphology of the signals. Thus, there is a tendency for certain sections of waveforms to be oversmoothed, or over-filtered, and/or artificial distortions such as waveform discontinuities to be created. Hence, conventional signal processing can introduce artificial waveform morphology into a processed signal which can result in misdiagnosis or improper therapy.
Thus, there is a need for a signal processing scheme that can provide noise filtering while preserving signal morphology. There is a further need for a signal processing scheme that can provide real-time adaptive filtering, particularly of low amplitude, high frequency signal components from a continuous stream signal, while leaving high amplitude features of the signal intact at all frequencies.